1. Field of the Invention
This invention relates to improved catalysts, their manufacture, and use in hydrocarbon synthesis, Fischer-Tropsch type reactions, to prepare heavy hydrocarbons, C.sub.5+, from synthesis gas, carbon monoxide and hydrogen.
2. The Prior Art
Hydrocarbon synthesis, Fischer-Tropsch, reactions involving the reaction of carbon monoxide and hydrogen over a catalyst, to produce heavier hydrocarbons, C.sub.5 +, are diffusion limited. Hydrogen diffuses at a faster rate into the catalyst particle than carbon monoxide and the H.sub.2 /CO ratio at any point below the exterior profile of the catalyst will be greater than the hydrogen to carbon monoxide ratio in the bulk gas. As the reactants, hydrogen and carbon monoxide, proceed further into the catalyst particle towards its center, the ratio of hydrogen to carbon monoxide will continue to increase as long as the catalyst metal sites of the catalyst are available to consume H.sub.2 and CO.
The stoichiometric consumption ratio of hydrogen and carbon monoxide to produce heavy hydrocarbons is 2.1/1. Ratios above the stoichiometric ratio favor increasing formation of methane, CH.sub.4. However, methane is not a desirable product of the hydrocarbon synthesis reaction, although the synthesis gas is often prepared from methane, for example, by partial oxidation or catalytic reforming.
Various methods for overcoming the diffusion limitation phenomenon of hydrocarbon synthesis catalysts have been developed in the prior art, one of which is the coated or rim-type catalysts. Examples of coated or rim type catalysts are U.S. Pat. Nos. 2,512,608 and 4,599,481, and 4,637,993. The latter two patents disclose cobalt or promoted cobalt catalysts on supports which may be silica, alumina, or silica-alumina. Other examples of rim-type catalysts appear in Everson et al, Fischer-Tropsch Reaction Studies with Supported Ruthenium Catalysts, Journal of Catalysis, 53, 186-197 (1978) and Dixit and Tavlarides, Kinetics of the Fischer-Tropsch Synthesis, Ind., Eng. Chem. Proc. Des. Dev. 1983, 22, 1-9, where various metals such as cobalt, ruthenium, and iron are disclosed as hydrocarbon synthesis catalysts and a coated ruthenium on alumina catalyst is utilized in hydrocarbon synthesis rate studies. The catalyst has a rim of about 300 um in thickness.
The use of coated or rim type catalysts has been successful in limiting methane formation and increasing C.sub.5 + yields in hydrocarbon synthesis reactions. However, the preparation of rim type catalysts is relatively difficult and the use of available catalyst metal atoms as sites for promoting the desired reactions has not necessarily been efficient.
In addition, during the reduction of the catalyst metal presursor, which typically requires heating in the presence of a reducing agent, the catalyst precursor is not heated at a single steady heating rate. In the prior art, heating is carried out over a broad range of temperatures for a specified time to maximum temperature. For example, Farrow et al U.S. Pat. No. 2,696,475 teaches a method for preparing nickel, cobalt or copper catalyst using a hydrogen reducing agent. The catalyst precursor is heated from 300.degree. C. to 500.degree. C. for about 4 hours. In Mauldin U.S. Pat. No. 4,568,663 a rhenium promoted cobalt catalyst precursor is reduced at temperatures ranging from 250.degree. C. to 500.degree. C. for periods ranging from 0.5 to 24 hours. In neither case is a particular heating rate for the catalyst precursor material specified.
The rate of heating during reduction later became more important after it was recognized that the reduction of the catalyst metal precursor to improve catalyst activity was temperature path dependent. However, these processes either employ a multiplicity of heating rates or calcine the catalyst at high temperatures before the reduction step. Reducing the catalyst metal precursor directly, without calcination, was then known to result in catalysts having inferior activity than that of catalysts produced by a process that employs a calcination step.
For example, Eri et al U.S. Pat. No. 4,801,573, shows a method for preparing a rhenium and cobalt catalyst on a support. The reduction is carried out at the single steady rate of 1.degree. C./min. to the maximum temperature of 350.degree. C. However, the process requires calcination of the catalyst precursor before reduction. Koblinski et al U.S. Patent 4,605,676, reveals a process for directly reducing cobalt catalyst precursors without calcining. The catalyst precursors are prepared from cobalt nitrate solutions and cobalt carbonyls on alumina based supports. During the reduction step, the heating rate is held steady at 1.degree. C./min. to 350.degree. C. For comparative purposes, the directly reduced catalyst (R350) was subjected to an additional oxidation and reduction step to produce a second catalyst (ROR). Koblinski teaches that the catalyst prepared by the ROR procedure has much higher activity than catalyst prepared by direct reduction (R350) at a heating rate of 1.degree. C./min.
Beuther et al In U.S. Pat. No. 4,493,905, gas synthesis catalysts are prepared by directly reducing the catalyst from a catalyst metal salt. Although calcination was omitted, the reduction was carried out by a multiple of heating steps of different rates, each to a different maximum temperature. For instance, in one embodiment, the catalyst is initially heated at 1.degree. C./min. up to 100.degree. C. and held at that temperature for about 1 hour and then heated at 1.degree. C./min. up to 200.degree. C. and held at that temperature for about 2 hours and finally, heated at 10.degree. C./min. up to 360.degree. C. and held at that temperature for 16 hours.
We have now discovered a method for preparing coated or rim type catalysts having a relatively uniform rim of predictable thickness and wherein the efficiency of the catalyst metal atoms and promotion of desired reactions is greatly enhanced. Furthermore, we have discovered a process whereby catalyst dispersion and activity can be increased by directly reducing a catalyst metal precursor, in the form of a salt, in the presence of a reducing agent where heating during the reduction is a single steady heating rate up to a maximum temperature. The process is applicable to rim type as well as non-rim type catalysts, e.g., powdered catalyst.